In xerographic copiers, the image of a document or object is latently formed on a photoconductor and developed by application of toner to the photoconductor. The image as represented by the toner pattern is subsequently transferred to a copy sheet. The copy sheet then passes through a fuser which fixes the image in substantially permanent form on the copy sheet. The present invention is concerned with heated such fusing devices and is especially useful in copiers employing heated rollers for the fusing purpose. Although this invention is not limited to the specific structural environment shown therein, U.S. Pat. No. 4,162,847 entitled "Hot Roll Fuser Early Closure Inhibitor" by F. Y. Brandon issued July 31, 1979 and assigned to the same assignee as this application, illustrates one example of a copier environment and hot roll fuser structure.
Minimizing the warm-up time of a heated fuser in a copier is particularly desirable to allow the operator to use the copier as soon as possible after it has been initially turned on. However, it is important to avoid driving the fuser temperature excessively beyond the desired operating temperature because this results in fusing problems as well as reduced life of the fuser.
Early fuser control systems merely employed a predetermined time-out to prevent machine operation until more than adequate time had passed for the fuser to have reached full operating temperature by applying full power to the fuser until the time-out has ended. Typically, the fuser is allowed to return to a standby temperature somewhat lower than the original warm-up temperature. Clearly this procedure required unnecessary delays in availability of the copier. The fixed-time out devices are particularly undesirable where the machine operation is temporarily interrupted as for jam clearances and power immediately returned to the machine. In such cases, the fuser temperature typically is only somewhat below the normal operating temperature and application of warm-up power results in excessive heating at the fuser. Of course, thermal relays are applicable to stop power application when the temperature reaches an excessive level, but the time-out process continues.
Prior art techniques of fuser temperature control include allowing the fuser temperature to cross an operating point twice before permitting copier use. Full power is applied to the fuser upon power initiation until the temperature sensor reaches the desired operating point. The temperature continues to rise overshooting the desired operating temperature. Power is reapplied once the temperature has descended below the operating point a second time and it is then assumed the copier is ready for use. Overshoot is minimized by this procedure but warm-up time is not significantly reduced.
Yet another prior art technique involves introduction of a control temperature below the operating point. Full power is applied to the fuser until the temperature reaches this lower temperature level. Power is then reduced to a set amount. The fuser temperature sensor reflects a movement of the temperature to the operating point. This process minimizes overshoot, but the coasting time added to the total warm-up time, is still significant. Further, if the copier is interrupted in normal operation as for jam clearances so that the temperature drops to a point between the lower level and the operating point, application of full power to the fuser upon repowering of the copier is not possible. This lengthens the waiting time after jam clearances.
Arrangements for fuser temperature control using appropriately coupled circuitry and computers, microprocessors and the like are well known. For instance, coupling of a hot roll sensor into a bridge detector with the voltage difference at the bridge being analog-to-digital converted into a microprocessor for control of a triac as to its time division power-on basis is known. An example of such a system wherein half cycles of triac on-time are controlled is shown in the cross-referenced Cunningham and Gianos application. The Cunningham and Gianos patent shows a processor output of digital data into a comparator for comparison against the digital output of the analog-to-digital converter from the bridge.
Another prior art arrangement employs a thermistor coupled into a bridge for a microprocessor input. Typically the fuser is driven to a high level during the warm-up period, a low level during standby and an intermediate level during copying operations. Again this is performable by half cycle controls of triacs or the like. An example of such a system is shown in U.S. Defensive Publication No. T100804 entitled "Microprocessor Controlled Power Supply for Xerographic Fusing Apparatus" by L. M. Ernst published July 7, 1981 (1008 O.G. 1).
The prior art does not disclose methods and apparatus for both warm-up time and temperature overshoot minimization but this result is advantageously provided by the present invention.